Residual-current circuit breaker and a method for testing the reliability performance of a residual-current circuit breaker

ABSTRACT

A test circuit is provided, which is independent of a power supply and which enables a circuit breaker to be tested in a reliable manner. The test circuit includes a test coil which is devoid of electric potential and is wound around a totalizing current transformer. The test coil is preferably short circuited using a test switch and a connectable load. This simulates the occurrence of a residual current. The selection of an appropriate connectable load allows the sensitivity of the current circuit breaker to be tested in an advantageous manner.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP01/04618 which has an Internationalfiling date of Apr. 24, 2001, which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The invention generally relates to a fault-current circuit breaker. Inparticular, it relates to a differential-current circuit breaker, havinga core-balance current transformer and a control winding wound aroundit. The invention also generally relates to a method for checking thereliability of such a fault-current circuit breaker.

BACKGROUND OF THE INVENTION

Fault-current circuit breaker are used in electrical systems, in orderto protect personnel against dangerous body currents. When a faultcurrent occurs, the circuit breaker disconnects the conductors of aconductor network. The circuit breaker is used as an autonomous unit, orelse as an additional module for a switching device. Such an additionalmodule is referred to as a circuit breaker accessory.

With regard to circuit breakers, a distinction is drawn between mainvoltage independent, so-called FI switches (fault-current circuitbreakers) and main-voltage-dependent DI circuit breakers(differential-current circuit breakers). Both switch types have acore-balance current transformer, through which the conductors of aconductor network are passed. A control winding is wound around thecore-balance current transformer and is connected to an evaluation unit,via which a release is actuated. When an unacceptable fault currentoccurs in the conductor network, this is detected by the core-balancecurrent transformer with the associated evaluation unit, and the releasedisconnects the conductors of the conductor network via a switchingmechanism. The fault current at which the circuit breaker responds isreferred to as the tripping fault current. The ratio of the trippingfault current to the so-called rated fault current is fixed and isdefined by Standards for the various fault current types. The ratedfault current is a measure of the protection class for which therespective circuit breaker is designed.

As a rule, FI/DI circuit breakers have a test device, by which thereliability of the circuit breaker can be checked. Such a test devicenormally connects two primary conductors to one another via a seriescircuit comprising a test resistor and a test winding and via a testcontact (pushbutton) which can be operated, forming a test circuit. Afault current is thus produced when the test contact is closed, and thisis detected by the core-balance current transformer together with theassociated evaluation unit. A test device such as this can be found, forexample, in the article “Fehlerstrom-Schutzschalter zum Schutz gegengefährliche Körperströme”, [Fault current circuit breaker for protectionagainst dangerous body currents] etz, Volume 110 (1989), Issue 12, pages580–584.

When two conductors are short-circuited for test purposes, there is aproblem in that, in some circumstances, a test current will flowpermanently via the test circuit for as long as the test contact isoperated. This problem occurs when connection of the test circuit to theconductors of the conductor network is made on the mains voltage supplyside, that is to say upstream of the circuit breaker switchingmechanism, so that current continues to flow in the test circuit evenafter the conductors have been disconnected by the circuit breakerduring the test. Thus, in conventional test devices, an auxiliary switchwhich is coupled to the switching mechanism of the circuit breaker isoften connected in the test circuit, and interrupts the test circuitwhen the circuit breaker trips, in order to ensure that the current flowis reliably interrupted. However, the arrangement of an auxiliary switchon the one hand requires additional measures and on the other hand isnot always possible, for example in the case of FI/DI accessories, sincethere is no switching mechanism for space reasons. If it is impossibleto arrange any auxiliary switches, the test circuit must therefore bedesigned, for example, for permanently flowing test current. The designfor permanent excitation is extremely complex, in particular when thecircuit breaker is designed for high-rated fault currents.

SUMMARY OF THE INVENTION

An embodiment of the invention is based on an object of specifying afault-current circuit breaker and/or a method for checking itsreliability which allows the circuit breaker to be configured in asimple and functionally reliable manner.

An object with regard to the circuit breaker can be achieved accordingto an embodiment of the invention by a fault-current circuit breaker, inparticular a differential-current circuit breaker, with a core-balancecurrent transformer and with a control winding wound around it. Amain-voltage-independent test circuit is preferably provided, with afloating test winding wound around the core-balance current transformer.

In contrast to the known test device, in which an actual fault currentis produced by short-circuiting two conductors of the conductor network,an embodiment of the invention can be based on the idea of justsimulating a fault current. This avoids the power loss problemsassociated with a test current flowing for an undefined time. Thecritical element for simulation of a fault current is the floating testwinding, that is to say a coil which is wound around the core-balancecurrent transformer and has no connection for the conductors of theconductor network.

The simulation is based on the principle that the magnetization of thecore-balance current transformer is varied by the induction principle asa function of the short-circuit resistance of the test winding. Thiseffect also occurs in the case of a fault current, in this case, as themagnetic fields of the conductors which are passed through thecore-balance current transformer no longer cancel one another out. Thechange to the magnetization of the core-balance current transformeris—as is normal for all-current-sensitive DI circuit breakers—detectedby the control winding, which is stimulated by an AC voltage, and by theassociated evaluation unit. The evaluation process is in this casecarried out essentially on the basis of the permeability, which can bemeasured or determined via the control winding, of the core-balancecurrent transformer, and is dependent on the change in magnetization ofthe core-balance current transformer.

In one preferred embodiment, the test circuit has a test switch viawhich the test winding can be short-circuited. This allows the testcircuit to be designed with particularly simple circuitry. Theexpression test switch also means, in particular, a test button.

The test circuit is in this case preferably designed without a separatevoltage supply. The test is thus carried out in a floating manner, inthe sense that there is no specific voltage source. In fact, it issufficient to use for the test the voltage which is induced in the testwinding by the alternate magnetization of the core-balance currenttransformer by the control winding.

In one particularly expedient refinement, the test circuit has a burdenwhich can be connected and which is used to influence the permeabilitywhich can be measured via the control winding. The choice of the burdenin this case advantageously makes it possible to select the permeabilitywhich can be measured, thus simulating a specific fault-current level.The arrangement of the burden thus makes it possible to check thesensitivity of the circuit breaker.

The burden is in this case preferably formed by a resistor connected inseries with the test switch. If the test circuit is in the form of ashort-circuiting circuit, the short-circuit is thus produced via theresistor, which is then arranged in parallel with the test windings.

In one particularly expedient refinement, the burden which can beconnected is designed such that, when the test contact is closed atripping criterion which is predetermined for the circuit breaker issatisfied, or is more than satisfied by a defined amount.

As already mentioned, the burden offers the capability to check thesensitivity of the circuit breaker. If the burden which can be connectedis designed such that the tripping criterion which is predetermined forthe circuit breaker by Standards is satisfied exactly, it is possible toidentify rises in the tripping fault current above the permissible limitvalue. If the burden is chosen such that the tripping criterion is morethan satisfied by a specific amount, it is possible to ensure correcttripping even in the event of poor component tolerances. The trippingcriterion would be exceeded by several times in the case of a pureshort-circuit winding with a resistance of zero ohms. A test such asthis thus relates to a pure functional test of mechanical disconnectionof the conductors. The choice of the burden for determining the trippingcriterion provided for the circuit breaker in this case depends on thedesign of the circuit breaker, for example, on the number of windingsfor the control winding.

With the previous method of short-circuiting two conductors, it wasimpossible to check the sensitivity, or this could be carried out onlyto a highly restricted extent, since the rated voltage range is normallywide. This is because, with a conventional test device, insensitivity ofthe circuit breaker remains undetected up to a certain level. This isbecause there is a risk of the circuit breaker not tripping at thespecified tripping fault current, but only at a multiple of it, as aresult of a functional defect. A functional check based on theconventional method would not detect this functional defect, since thetripping fault current would be considerably exceeded byshort-circuiting of the conductors. The test method provided by thefloating test winding thus allows considerably better results to beobtained with respect to the serviceability of the circuit breaker, thana conventional test device. In particular, there is no risk of afunctional defect remaining undetected and, in the worst case, injuringsomeone if a fault current were to occur.

In one advantageous embodiment, the burden which can be connected isvariable, in particular in order to make it possible to check differentsensitivities for circuit breakers whose rated fault current isadjustable. The variability of the burden is achieved, for example, byusing a variable potentiometer in the test circuit, or else by usingdifferent resistors, for example in conjunction with a multi-stagerotary switch in the test circuit.

According to one particularly expedient refinement, the test circuit hasa continuously acting burden, which influences the permeability whichcan be measured via the control winding.

In conventional circuit breakers, such a permanent burden is oftenarranged in parallel with the control/or secondary winding, in order todefine the tripping response of the circuit breaker. However, this hasthe disadvantage that a current flows via the burden, which is arrangedin parallel to the control winding, and this makes it harder to evaluatethe voltage drop across the measurement resistor, which is connected inseries with the control winding, as a measure of the measuredpermeability. With regard to the tripping response, the arrangement ofsuch a permanently acting burden in the test circuit has the same effectas the arrangement in parallel to the control winding, but offers themajor advantage that it considerably simplifies the evaluation of thevoltage drop across the measurement resistor, which is connected inseries with the control winding.

The permanently acting burden is preferably available, so that it ispossible to set the tripping fault current and/or the rated faultcurrent. In conjunction with the capability to check different trippingfault currents by means of the variable burden which can be connected,it is thus possible to ensure correct tripping even with poor componenttolerances.

The test winding is preferably wound symmetrically around thecore-balance current transformer. The symmetrical or uniform windingaround the core-balance current transformer in this case ensures thatthe resultant inductive effect of the inhomogeneous magnetic fieldscaused by the load currents is cancelled out overall, and that nodisturbance voltages are thus induced in the winding. This isparticularly necessary when there is a permanently acting burden in thetest circuit since this results in the core-balance current transformerbeing burdened independently of the field distribution. The uniformwinding results in non-uniform field distributions in the transformercore, for example caused by the dipole field of the load current, beingaveraged out over the circumference of the core-balance currenttransformer.

In one expedient refinement, the test circuit has an additional switchor push button in order to trip the circuit breaker remotely, via whichthe test winding can be short-circuited.

In particular, the floating configuration of the test circuit isadvantageous with regard to the safety requirements for such remotetripping. At the same time, the test winding is expediently sufficientlywell isolated from the control winding, which is electrically connectedto the conductor network, in order to satisfy the safety requirements,which demand safe conductive isolation between a remote tripping circuitand the conductor network.

In one preferred alternative, a further winding is provided around thecore-balance current transformer, in addition to the test winding, forremote tripping. This further winding can preferably likewise beshort-circuited. If the circuit for remote tripping has no permanentlyacting burden, a small number of turns in the further winding aresufficient to ensure operation of the remote tripping. In this case, thewinding need not be designed to be symmetrical. This has the advantagethat it simplifies the isolation of the control winding.

According to one embodiment of the invention, the reliability of afault-current circuit breaker can be simulated by a test circuit havinga test winding wound around the core-balance current transformer.

The advantages and preferred embodiments mentioned with regard to thecircuit breaker can be transferred in the same sense to the method.Particularly expedient refinements of the method are specified in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be explained in more detailin the following text with reference to the drawings in which, in eachcase illustrated schematically:

FIG. 1 shows a circuit diagram of a circuit breaker with a test circuit,

FIG. 2 shows a circuit diagram of a circuit breaker with a modified testcircuit and a separate remote tripping circuit,

FIG. 3 shows a B-H diagram with different magnetization curves,

FIG. 4 shows a detail from the circuit arrangement of a circuit breakerwith a burden arranged in parallel with the control winding of thecircuit breaker, and

FIG. 5 shows a detail of a circuit arrangement of a test switch with apermanent burden arranged in parallel with the test winding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a fault-current circuit breaker 2 has a core-balancecurrent transformer 4, a control winding 6 wound around it, and afunctional unit 8. The latter not only has actuation and evaluationelectronics for the control winding 6, but also a tripping mechanism.The conductors L1, L2, L3 and the neutral conductor N of a conductornetwork are passed through the core-balance current transformer 4. Eachconductor L1–L3, N has an associated interrupter switch 10, via whichthe conductors L1–L3, N are disconnected by means of a switchingmechanism 12, which is represented by dashed lines, when an unacceptablefault current occurs.

Supply lines 14 lead from the individual conductors L1–L3, N to thefunctional unit 8, in order to provide a power supply for theelectronics integrated in it. The circuit breaker 2 as shown in FIG. 1is thus, by definition, in the form of a main-dependent DI circuitbreaker.

In addition to the already described elements, which every DI circuitbreaker 2 has, the circuit breaker as shown in FIG. 1 has, as anessential new feature, a test circuit 16 with a test winding 18 woundaround the core-balance current transformer 4. A permanently actingburden R1 in the form of a resistor is provided in parallel with thetest winding 18. The test circuit 16 has a test switch 20, via which thetest winding 18 can be short-circuited via a further burden R2 which canbe connected. The latter is likewise in the form of a resistor, which isarranged in series with the test switch 20. In the exemplary embodimentshown in FIG. 1, the test winding 18 is at the same time part of aremote tripping circuit 22, which is connected via a remote trippingline 24 to the test circuit 16 and has a switch 26 which is arranged inparallel with the test switch 20.

For safety reasons, and in order to ensure that the circuit breaker isresistant to surge currents, a voltage-limiting element may be providedin parallel with the control winding and/or in parallel with the testwinding.

The exemplary embodiment of a circuit breaker 2 shown in FIG. 2 differsfrom that shown in FIG. 1 in that the remote tripping circuit 22 is inthe form of a separate remote tripping circuit 22 with its own winding30, and in that the permanently acting burden R1 and the burden R2 whichcan be connected are configured as variable resistors, in the form of adouble potentiometer. The remote tripping circuit 22 can in this case beshort-circuited via the switch 26 and via a resistor which acts as aburden R3, in order to cause the circuit breaker R2 to trip.

The method of operation of the test circuit 16 for checking thereliability of the circuit breaker 2 will be explained in conjunctionwith FIG. 3 in the following text. FIG. 3 shows a B-H diagram,illustrating a number of magnetization curves I–IV. The magneticinduction B is plotted on the ordinate, against the magnetic fieldstrength H on the abscissa. The individual magnetization curves I–IVhave different gradients, with the magnetization curve I bendingconsiderably into a saturation region above a specific magnetic fieldstrength H. The gradient of the individual magnetization curves I–IVcorresponds to the permeability μ, as detected by the control winding 6,of the core-balance current transformer 4. The permeability μ measuredby the control winding 6 is governed by the actual permeability of thecore-balance current transformer 4 and by superimposed effects. Onesuperimposed effect, by way of example, is the occurrence of a faultcurrent in the conductor network, or else a burden. Both effects cause achange to the profile of the magnetization curve and are detected by thecontrol winding with the associated evaluation unit. The permeability μ,that is to say the gradient of the magnetization curve, is generally setby the permanent burden R1. In this case, the gradient of themagnetization curve becomes ever flatter, as the resistance of theburden R1 is decreased. The tripping response of the circuit breaker 2is also governed by the permanent burden R1.

An AC voltage is applied to the control winding 6, so that thecore-balance current transformer 4 is magnetized alternately. Themagnetization curve is in this case evaluated at an operating point Hafor a defined magnetic field strength H. This makes use of the fact thatthe coil resistance of the control winding 6 is high when thepermeability μ is high, and is correspondingly reduced when thepermeability is less. The voltage drop across the control winding 6 isevaluated via a measurement resistor 28 (in this context, see FIG. 4 andFIG. 5).

The test winding 18 is terminated via the permanently acting burden R1.The alternate magnetization of the core-balance current transformer 4via the control winding 6 results in a voltage being induced in the testwinding 18, so that a current flows in the test circuit 16 which resultsin the test winding 16 producing a magnetic field which counteracts themagnetization of the core-balance current transformer 4 caused by thecontrol winding 6. The permeability μ measured by the control winding 6is thus less than the actual permeability of the core-balance currenttransformer 4.

When the test switch 20 is operated, the further burden R2 is connected,so that the measurable permeability μ is changed once again. If theresistance of the further burden R2 is in this case reduced, thisresults in a greater change in the permeability μ. The burden R2 whichcan be connected is now preferably chosen such that the change caused inthis way to the measurable permeability μ corresponds to the situationwhen a fault current occurs, for example a tripping fault current, inresponse to which the circuit breaker 2 disconnects the conductorsL1–L3, N. The connection of the burden R2 therefore simulates theoccurrence of a fault current.

The major advantage of this test method is that the choice of a suitableresistance for the burden R2 makes it possible to simulate trippingfault currents of different magnitudes so that it is possible to checkthe sensitivity of the circuit breaker 2. Furthermore, the test circuit16 does not require a separate voltage supply. This is because itsprinciple of operation results in a voltage being induced via the testwinding 18 in the test circuit 16.

A further advantage of the test circuit 16 is that it can at the sametime be used for remote tripping. This can be done just by connectingappropriate remote tripping lines 24 to the test circuit 16. Inparticular, the floating configuration of the test circuit 16 isadvantageous with regard to the safety requirements for such remotetripping. If the test circuit 16 is at the same time used for remotetripping, then it is necessary to ensure that the test winding 18 issufficiently well isolated from the control winding 6, which is normallyat the same potential as the main circuit.

If the permanent burden R1 is arranged in the test circuit 16 as shownin FIG. 1, the test winding 18 is preferably arranged symmetrically anduniformly around the core-balance current transformer 4. This results inthe core-balance current transformer 4 being burdened independently of afield distribution, in order to avoid errors in the evaluation resultingfrom inhomogeneities in the magnetic fields. Such inhomogeneities arecaused by an asymmetric arrangement of the conductors L1–L3, N in thecore-balance current transformer 4 so that, even when no fault currentis flowing, local magnetic fields occur which lead to localmagnetization in the core-balance current transformer 4. The inductioneffects of these local magnetizations cancel one another out overallonly when a winding is distributed homogeneously on the transformercore.

The advantageous arrangement of the permanent burden R1 in parallel withthe test winding 18, instead of the arrangement in parallel with thecontrol winding 6, will be explained with reference to FIG. 4 and FIG.5.

FIG. 4 in this case shows the conventional arrangement of the burden R1in parallel with the control winding 6, and FIG. 5 shows the newarrangement of the permanent burden R1 within the test circuit 16. An ACvoltage is applied to the control winding 6 via a voltage generator 32.The already mentioned measurement resistor 28, which is used to detectthe voltage drop across the control winding 6 as a measure of themeasurable permeability, is in each case arranged in series with thetest winding 6. An evaluation circuit 34 is provided in parallel withthe measurement resistor 28. In the arrangement shown in FIG. 4, acurrent element I1 flows via the control winding 6, and a currentelement I2 flows via the permanent burden R1. The voltage drop U acrossthe measurement resistor 28 is governed by the two current elements I1,I2. In contrast to this, and according to the exemplary embodiment shownin FIG. 5, all the current I1′ flows via the control winding 6. Thissimplifies the evaluation of the voltage drop U across the measurementresistor 28.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A fault-current circuit breaker, comprising: a core-balance currenttransformer; a control winding wound around the core-balance currenttransformer; and a main voltage independent test circuit, including afloating test winding wound around the core-balance current transformer,a burden connected in parallel to the floating test winding, and afurther burden, adapted to be connectable and influence the permeabilityof the core-balance current transformer, measurable via the controlwinding.
 2. The circuit breaker as claimed in claim 1, wherein the testcircuit includes a device adapted to short circuit the test winding. 3.The circuit breaker as claimed in claim 1, wherein the test circuit doesnot include a separate voltage supply.
 4. The circuit breaker as claimedin claim 1, wherein the further burden includes a resistor connected inseries with a test switch.
 5. The circuit breaker as claimed in claim 1,wherein the further burden is designed such that a tripping criterion,which is predetermined for the circuit breaker, is at least satisfied bya defined amount.
 6. The circuit breaker as claimed in claim 1, whereinthe further burden is variable.
 7. The circuit breaker as claimed inclaim 1, wherein the burden is continuously acting to influence thepermeability of the core-balance current transformer, measurable via thecontrol winding.
 8. The circuit breaker as claimed in claim 7 whereinthe continuously acting burden is variable.
 9. The circuit breaker asclaimed in claim 1, wherein the test winding is wound symmetricallyaround the core-balance current transformer.
 10. The circuit breaker asclaimed in claim 1, wherein the test circuit includes a switch, viawhich the test winding can be short-circuited, for remote tripping. 11.The circuit breaker as claimed in claim 1, wherein a further winding isprovided around the core-balance current transformer, for remotetripping.
 12. The fault-current circuit breaker of claim 1, wherein thefault-current circuit breaker is a differential current circuit breaker.13. The fault-current circuit breaker of claim 2, wherein the device isat least one of a test switch and push button.
 14. The circuit breakeras claimed in claim 2, wherein the test circuit includes a furtherburden, adapted to be connectable and influence the permeability of thecore-balance current transformer, measurable via the control winding.15. The circuit breaker as claimed in claim 14, wherein the furtherburden includes a resistor connected in series with the test switch. 16.The circuit breaker as claimed in claim 4, wherein the further burden isdesigned such that a tripping criterion, which is predetermined for thecircuit breaker, is at least satisfied by a defined amount.
 17. Thecircuit breaker as claimed in claim 14, wherein the further burden isdesigned such that a tripping criterion, which is predetermined for thecircuit breaker, is at least satisfied by a defined amount.
 18. Thecircuit breaker as claimed in claim 4, wherein the further burden isvariable.
 19. The circuit breaker as claimed in claim 5, wherein thefurther burden is variable.
 20. A method for checking the reliability ofa fault-current circuit breaker including a core-balance currenttransformer and a control winding wound around the core-balance currenttransformer, comprising: simulating an occurrence of a fault current bya main voltage independent test circuit with a floating test windingwound around the core-balance current transformer, and a burdenconnected in parallel to the floating test winding; and short-circuitingthe test winding via a connectable further burden, so that permeabilityof the core-balance current transformer, measurable via the controlwinding, assumes a defined value.
 21. The method as claimed in claim 20,wherein the burden is variable.
 22. The method as claimed in claim 20,wherein the circuit breaker is a differential-current circuit breaker.23. The method as claimed in claim 20, wherein the further burden isvariable.